We propose to study how protein environment and/or chromophore interaction controls the properties and reactivity of a chromophore using resonance Raman spectroscopy by examining siroheme and iron-sulfur clusters in different sulfite reductases from sulfate reducing bacteria. Resonance Raman spectroscopy can yield detailed molecular information of the chromophore in situ unavailable by other methods. Sulfite reductases catalyze the 6-electron reduction of sulfite to sulfide. The reduction is crucial for synthesis of sulfur-containing cell constituents and in terminal respiration for sulfate reducing bacteria. Although sulfite reductases contain the same two chromophores, their properties are quite different, making them ideal systems for studying structural changes with reactivity. In order to understand the Raman spectra of the chromophores, we plan to study the chromophores and their analogs in various solutions and environments. We also propose to study the mechanism of sulfite reductases by examining changes in the chromophores during catalysis, at different redox potentials, and bound to different substrates. We also propose to characterize the chromophores of other iron- sulfur proteins from sulfate reducing bacteria.